Abstract:
African horse sickness (AHS) is a devastating, non-contagious viral disease of equids. However, zebra are resistant to the disease. Previously it was accepted that outbreaks of AHS would start in the northern parts of South Africa and gradually move southwards as the season progressed. This stemmed from the belief that the large population of zebras in the Kruger National Park, acted as the source of the virus. In outbreaks that have occurred over the past 25 years, it has become evident that this was not true, as outbreaks would persist for two or more seasons in the south of the country, with the same AHS serotype causing the first outbreak being the only one recorded in that area in subsequent years. Therefore, it is possible that the virus overwinters in the area. However, the overwintering mechanism of the African horse sickness virus (AHSV) is not fully understood. It has been suggested that the AHSV overwinters in adult Culicoides populations, at low levels of viremia in natural hosts, or in other vertebrate or invertebrate hosts such as dogs and ticks, respectively. The aim of this study was to investigate how the AHSV overwinters in the study area, and the role dogs and ticks play in this process and to determine the diversity for both Culicoides and tick species in the study area. This study was undertaken in the Sarah Baartman district in the Eastern Cape Province of South Africa. A total of 28 sites were recruited for collection of Culicoides and tick species, and blood samples from dogs. A 220 V down draught-UV light trap was used to collect Culicoides specimens, while ticks were collected directly from live hosts. Blood samples were drawn from dogs using the cephalic vein. All these samples were sent to the Agricultural Research Council – Onderstepoort Veterinary Research (ARC-OVR) for analysis. Over the survey period 918 collections of Culicoides were made, and 44,850 Culicoides specimens, comprising of 49 Culicoides species were recovered. A total of 1,260 tick specimens comprising of 10 species from three genera were collected and identified during the survey. Over a period of one year, blood samples were collected from 100 dogs. Each dog was sampled four times, once every season. Blood samples were analysed using an AHS indirect ELISA to test for the presence of IgG antibodies. Culicoides and tick samples were first tested for the presence of AHSV using an inhouse PCR. All samples that tested positive on the inhouse PCR were subjected to the WOAH (accredited and validated) test. Only samples that tested positive with the WOAH test were considered AHSV positive. While five Culicoides pools tested positive with the inhouse PCR, only two pools of these pools tested positive for the virus on the WOAH PCR test. Furthermore, of the two pools that tested positive: one was from Site 1 and consisted of two specimens of C. bolitinos and the other pool was from Site 7 and consisted of six specimens of C. tuttifrutti. This study provides an insight into the diversity of both Culicoides and tick species in the study area. Findings reported here demonstrated that dogs are not preferred hosts for Culicoides species given that none of the dogs tested positive for AHSV antibodies. Based on the results from this study, only Culicoides species have the potential to play a role in the overwintering of AHSV in the area. However, additional tick species and other possible vertebrate hosts should also be investigated. Further research is needed to investigate the role of C. tuttifrutti and other blood feeding flies in the transmission of AHSV and other orbiviruses.
